Donnerstag, 19. Januar 2017

One common misconception about thermodynamics and infrared radiation

Just another comment about the "there is no radiative greenhouse effect" meme caught my eyes.
It was in the comment section of a very informative interview with William Happer about climate change.

"Joseph E Postma says   

January 10, 2017 at 10:12 pm

There is in fact no radiative greenhouse effect at all. Climate science is so far off the mark that it is founded upon a concept which doesn’t even exist."

This was my answer to Joseph Postmas comment:

This argumentation does much harm to the skeptics issue. The simple fact is that nearly all matter above 0 Kelvin is radiating energy in form of infrared radiation, including CO2, water vapor and methane, which are members of the atmospheric gasses.

If theses gasses are radiating in all directions, whichever temperature above 0 K they have, a certain amount will go back to the earth's surface. Thereby they will slow the net transfer of heat from the surface to the space.

One common misconception is that " due to the laws of thermodynamic colder materials cannot heat up warmer materials, therefore colder CO2 cannot heat up the surface" - which in turn would insist that no colder item is allowed to send infrared radiation towards a warmer item.

The reality is: If a certain colder surface radiates 40 w/m² towards a warmer surface which radiates 100 w/m², the net flow is 60 w/m² towards the colder surface. This is exactly how thermodynamics work.

Just think about these gold or aluminium plated emergency blankets. If you wrap one around yourself in a cold surrounding you are getting warmer. The blanket is colder than you, but it radiates back some amount of heat towards your warmer body.

For a more elaborate explanation how radiation is working, just read at my blog:

For easy access here my example from that post:

  • There is a stove in a cold room, giving out a certain radiative energy.
  • Now take a black matte painted solid sheet around in a certain distance from the stove. It will be heated up and radiate towards to stove and the rest of the room. The stove will be hotter to a certain extend. This is what a cloud does, being close to the properties of  a blackbody.
  • Now take away the solid sheet and put there a black matte one with holes punched in, so that about two thirds of the area is covered. So this is now also heated by the stove, but to a lesser extend, and also the stove gets not so much hot than with the solid sheet. This is what water vapor is doing, leaving through some radiation.
  • Now take another black sheet with lots of holes punched in, covering only one quarter of the whole area. Now the re-radiation is much lower, and the stove stays only a small amount hotter. This is what CO2 is doing in the atmosphere.
  • Now try to get the whole picture: CO2 is always there in a quite even distribution. This is the sheet with the big holes. Water vapor is not evenly distributed. So it compares to a second sheet behind it,with differently big holes. Clouds are not always there, but if, they are covering the certian area completely.
  • We see: CO2 has a small function in the radiative play. So it doesn't really matter that much.
  • But: All three components of the atmosphere are re-radiating some amount of heat, thus slowing the cooling of the "stove". They are cooler than the stove and they are not heating up the stove. There is always a net flow of heat towards the cold room. Sometimes more, sometimes less, and always within the laws of thermodynamics.
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